The present invention relates generally to tuning systems and more particularly to all channel electronic television tuning systems having a time-sequenced counting system for determining the system tuning condition by counting the frequency of a tunable element and comparing that with information of a desired channel number. Specifically, it relates to a television tuning system incorporating a multispeed ramp or tuning voltage circuit for changing frequency at a different rate depending upon the proximity of the tuning system frequency to the desired frequency.
A paper by Doyle and Mills (BTR, Vol. 15, No. 2, July, 1969), discusses a channel selection and indication system (in block diagram form) employing a varactor tuner and means for detecting the received television signal and comparing its frequency with the channel number input information. The detection methods described are a frequency synthesizer and a marker (or birdy) synchronizing synthesizer. A comparator compares the incoming signal frequency with the input channel number information and, at equality, operates to send a stop signal to a tuner ramp voltage drive. A later paper by Doyle et al. (BTR, Vol. 18, No. 4, Nov. '72) shows an all channel television tuning system of the birdy frequency synthesizer type which includes an additional reference divisor to provide finer control of tuning when the tuner approaches the desired frequency by providing a change in counting sensitivity.
U.S. Pat. No. 3,798,553 to Sakamoto shows another television frequency sweep operation involving the alternate sweeping by two separate oscillators through the television frequency spectrum in a step-by-step motion. Voltage sweep circuits feed each of the two local oscillators and logic circuitry for providing the sequential advancement of the frequency.
The prior art discloses a number of digital tuning systems having "constant speed" voltage ramp driving circuits. Constant speed is used to indicate that the ramp voltage generated is essentially linear (generally determined by the charge or discharge of a capacitor) and thus produces a fairly uniform frequency sweep depending upon the capacity to tuning voltage characteristic of the varactor diodes. The speed of any ramp voltage system is, of course, governed by the response time of the detection system and a slow ramp speed to accommodate the detection system may introduce unacceptably long channel change times.